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Characteristics of diffusion-weighted images and apparent diffusion coefficients of ranulas and other masses in and around the floor of the mouth

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Characteristics of diffusion-weighted images and apparent diffusion coefficients of ranulas and other masses in and around the floor of the mouth

D1X XNao Wakasugi-Sato,D2X XDDS, PhD,aD3X XManabu Habu,D4X XDDS, PhD,bD5X XMasafumi Oda,D6X XDDS, PhD,a D7X XTatsurou Tanaka,D8X XDDS, PhD,aD9X XIkuko Nishida,D10X XDDS, PhD,cD11X XTetsuro Wakasugi,D12X XMD, PhD,d D13X XShinya Kokuryo,D14X XDDS, PhD,eD15X XDaigo Yoshiga,D16X XDDS, PhD,eD17X XTeppei Sago,D18X XDDS, PhD,f

D19X XNozomu Harano,D20X XDDS, PhD,fD21X XShinji Kito,D22X XDDS, PhD,aD23X XShinobu Matsumoto-Takeda,D24X XDDS, PhD,a D25X XTakaaki Jyoujima,D26X XDDS,aD27X XYuichi Miyamura,D28X XDDS,aD29X XNaomi Yada,D30X XDDS, PhD,g

D31X XMasaaki Sasaguri,D32X XDDS, PhD,bandD33X XYasuhiro Morimoto,D34X XDDS, PhDa

Objective. The aim of this study was to evaluate the characteristics of diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) values of ranulas. In addition, to elucidate DWI findings and ADC values of other representative masses in and around the floor of the mouth.

Study Design. DWI findings and ADC values in 35 patients with ranulas and 33 patients with other masses were retrospectively reviewed with a central focus on cystic masses or lesions that may have cyst-like components in and around the floor of the mouth based on the diagnosis of each respective disease.

Results. Ranulas were all well-defined, homogeneous masses with high signal intensity on DWI. The mean§ standard deviation ADC value of the 35 ranulas was 2.59§ 0.31 £ 10¡3mm2/s. There was a significant difference in ADC values between simple and plunging ranulas. On DWI, most other masses were heterogeneous, and most ADC values, except those for thyroglossal duct cysts, hemangiomas, and pleomorphic adenomas, were significantly lower than those for ranulas.

Conclusions. The characteristic DWI and ADC findings of ranulas can be determined accurately, and these data can be signifi- cantly useful in the differential diagnosis of many kinds of diseases in and around the oral floor. (Oral Surg Oral Med Oral Pathol Oral Radiol 2019;127:7784)

The utility of diffusion-weighted imaging (DWI) and apparent diffusion coefficients (ADCs) in magnetic resonance imaging (MRI) for the examination of acute cerebral ischemia is widely accepted in the medical field. DWI and ADCs are also thought to be of clinical value in the oral and maxillofacial region in the diag- nosis of conditions resulting from infectious diseases, neoplasms, trauma, and metabolic diseases.1-5

The area in and around the floor of the oral cavity is located close to the sublingual glands, nerves, tongue, and muscles that play important roles in digestion and speech. Many kinds of diseases, such as ranulas, epi- dermoid and thyroglossal duct cysts, hemangiomas, sublingual glandrelated tumors, carcinoma, and inflammatory lesions, originate in and around the oral floor. Differential diagnoses of these diseases can be very difficult because of the many kinds of tissues involved and the complex anatomy. In particular, if the lesions are relatively small, differentiating among the various lesions in and around oral cavity floor can be very difficult. MRI is very useful in the differential diagnoses of these lesions, including ranulas, because it has much higher contrast resolution between soft tis- sues and is a noninvasive modality without X-ray exposure.6Thus, DWI and ADCs have been clinically applied in the process of formulating differential diag- noses. There have been some reports of DWI findings

Statement of Clinical Relevance

The characteristic findings on diffusion-weighted imaging and the apparent diffusion coefficient val- ues of ranulas can be determined accurately, and these data can be significantly useful in the differen- tial diagnosis of many kinds of diseases of the oral floor.

This study was supported in part by grants-in-aid for scientific research from the Ministry of Education, Science, Sports and Culture of Japan to YM.

aDivision of Oral and Maxillofacial Radiology, Kyushu Dental Uni- versity, Kitakyushu, Japan.

bDivision of Maxillofacial Surgery, Kyushu Dental University, Kita- kyushu, Japan.

cDivision of Developmental Stomatognathic Function Science, Kyushu Dental University, Kitakyushu, Japan.

dDivision of Otorhinolaryngology-Head and Neck Surgery, Univer- sity of Environmental Health, Kitakyushu, Japan.

eDivision of Oral Medicine, Kyushu Dental University, Kitakyushu, Japan.

fDivision of Dental Anesthesiology, Kyushu Dental University, Kita- kyushu, Japan.

gDivision of Oral Pathology, Kyushu Dental University, Kitakyushu, Japan.

Received for publication Mar 16, 2018; returned for revision Aug 29, 2018; accepted for publication Sep 1, 2018.

Ó 2018 Elsevier Inc. All rights reserved.

2212-4403/$-see front matter

http://doi.org/10.1016/j.oooo.2018.09.002

77

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and ADC values of malignant and benign tumors.7-9 However, to the best of our knowledge, there have been no reports of the characteristic DWI findings and ADC values of ranulas and other masses in and around the floor of the mouth.

The purpose of the present study was to evaluate the characteristics of DWI and ADC values of ranulas. In addition, the characteristic DWI findings and ADC val- ues of other lesions were compared with those of ranu- las to facilitate the differential diagnoses of masses in and around the floor of the oral cavity, with a focus on lesions with cyst-like components.

MATERIALS AND METHODS

We conducted a retrospective investigation of patients who visited Kyushu Dental University Hospital from July 2011 to June 2016 and underwent MRI for lesions in and around the floor of the mouth. The sample popu- lation included 35 patients with ranulas (26 simple ranulas, 9 plunging ranulas; 18 males, 17 females;

mean age 32.1 years; age range 778 years) and 33 patients with masses other than ranulas (15 males, 18 females; mean age 53.5 years; age range 1192 years).

The lesions in the second group were cysts or cyst-like abnormalities located in (n = 17) and around (n = 16) the oral floor. These lesions included epidermoid cysts, thyroglossal duct cysts, hemangiomas, pleomorphic adenomas, squamous cell carcinomas, adenoid cystic carcinomas, and inflammatory diseases. The data from the patients with ranulas and from those with other masses are shown inTable I. A total of 23 ranulas and 11 mass lesions were evaluated microscopically on pathologic examination after surgical procedures. The clinical diagnoses of the cases that could not be diag- nosed microscopically were reported if surgical proce- dures were not performed. Eight simple ranulas, 4 plunging ranulas, 8 hemangiomas, 4 epidermoid cysts, 4 thyroglossal duct cysts, and 7 cases of inflammation were diagnosed clinically.

T1-weighted (T1WI), short-T1 inversion recovery (STIR), DWI, and ADC maps were obtained at the

MRI examinations. The imaging parameters used in these sequences are shown inTable II. Contrast MRI scans with gadolinium are acquired in our hospital only when malignant tumors are suspected. These scans are not obtained for benign lesions because of the risk of side effects caused by the contrast medium.

The 4 types of MRI images (T1WI, STIR, DWI, and ADC maps) mentioned above were obtained for all of the patients in the present study, whether the masses were benign or malignant. For the MRI examinations, verbal informed consent was obtained from all adult patients and from the parents or guardians of minors.

The Human Investigations Committee of Kyushu Dental University ensured that individuals’ rights were protected. Approval of the present study was obtained from the institutional review board of Kyushu Dental University (No. 13-19).

All images were acquired by using a 1.5-T full-body MR system (EXCELART Vantage Powered by Atlas;

Toshiba, Tokyo, Japan) with a circular polarized neck coil to visualize specific areas of the floor of the mouth.

DWI was performed by using single-shot multiecho par- allel echo planar imaging. Other scanning parameters are shown inTable II. Conventional single-section sagittal,

Table I. Pathologic and clinical diagnosis, age, and sex of 68 patients with cystic mass or lesions having cyst-like components in and around the floor of the mouth

Category Diagnosis Age (y) Male/Female

Pseudocysts (n = 35) Ranulas 32.1§ 19.3 18/17

Simple ranulas (n = 26) 33.8§ 21.8 11/15

Plunging ranulas (n = 9) 27.2§ 8.1 7/2

Cysts (n = 7) Epidermoid cysts (n = 4) 33.0§ 19.4 1/3

Thyroglossal duct cysts (n=3) 29.6§ 17.6 0/3

Benign tumors (n = 11) Hemangiomas (n=8) 47.9§ 19.9 4/4

Pleomorphic adenomas (n = 3) 70.0§ 5.3 0/3

Malignant tumors (n = 8) Carcinomas (n = 6) 63.0§ 11.0 5/1

Adenoid cystic carcinomas (n = 2) 78.5§ 9.2 2/0

Inflammation (n = 7) Inflammation (n = 7) 61.4§ 21.7 2/5

Table II. Imaging parameters

Sequences

T1WI STIR DWI

TR (ms) 820 4700 5500

TE (ms) 15 75 95

Flip angle/flop angle (˚) 90/160 90/160 90/180

FOV (mm) 200£ 256 200£ 200 200£ 200

Section thickness (mm) 6.0 6.0 6.0

Intersection gap (mm) 1.2 1.2 1.2

Slab thickness (mm) 7.2£ 15 7.2£ 15 7.2£ 15 Matrix (pixels) 224£ 320 272£ 272 144£ 144

B factor 1000

DWI, diffusion-weighted images; FOV, field of view; STIR, short-TI inversion recovery; T1WI, T1-weighted images; TE, time of echo;

TR, time of repetition.

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coronal, and transverse MRI scans of the oral and maxil- lofacial regions were obtained initially. The acquisition slab was oriented in the transverse direction on the sagit- tal and coronal images so that ranulas and other diseases in and around the oral floor could be included.

In patients with ranulas and other diseases of the floor of the mouth, characteristic DWI findings and ADC values were evaluated. ADCs were calculated, pixel by pixel, for areas with high signals indicative of ranulas on axial STIR images. In addition, ADC maps were prepared. Moreover, the extent, margins, internal architecture, and signal intensity were also investigated on T1-weighted and STIR images according to Kura- bayashi et al.6 The MRI scans were independently assessed by 2 research radiologists (N.W. and M.O.).

Both are expert radiologists: N.W. with 12 years of experience and M.O. with 11 years of experience are both specialists certified by the Japanese Society for Oral and Maxillofacial Radiology. The extent, mar- gins, internal architectures, and signal intensity on the MRI scans were judged by these 2 expert radiologists.

In case of disagreement, a final assessment was reached by consensus after discussion. All statistical tests, such as the Student t test for the analysis between 2 groups, Pearson’s correlation coefficient, and analysis of vari- ance (ANOVA) to compare more than 2 groups, such as ADC values and MRI findings between ranulas and other lesions, were performed by using the SPSS soft- ware, version 11 (SPSS Inc., Chicago, IL). The level of significance was P< .05. Correlations from Pearson’s analysis were classified into 5 grades: very weak, 0.00 to§0.20; weak, §0.21 to §0.40; moderate, §0.41 to

§0.70; strong, §0.71 to §0.90; and very strong, §0.91 to§1.00.

RESULTS

Characteristic findings on MRI, including DWI of ranulas and other lesions

On MRI, the 35 ranulas were well-defined homoge- neous masses with low signal intensity on T1WI and high signal intensity on STIR (Figure 1). On DWI, the 35 ranulas were relatively homogeneous, with high sig- nal intensity (see Figure 1). The 26 simple ranulas were all confined to the sublingual space and relatively small. The 9 plunging ranulas were centered on the submandibular space and were likely to spill into one or more adjacent spaces, such as the sublingual and parapharyngeal spaces. MRI findings, including DWI, such as extent, margins, internal architecture, and sig- nal intensity, of simple ranulas were similar to those of plunging ranulas (seeFigure 1;Table III). Plunging ranulas extended slightly into the sublingual space, the so-called tail sign, in 6 of 9 cases. There were no differ- ences in the MRI findings of ranulas between males

and females or between age groups, as illustrated in Figure 2.

The MRI findings of all other masses in the present study were different from those of ranulas in one or more ways. MRI findings of epidermoid cysts were mostly similar to those of ranulas. However, they were oval or round, unlike ranulas (Figure 3). For epidermoid cysts, no superior extension into the par- apharyngeal space was found in any case. In addi- tion, the signal intensity of one case on T1WI was medium intensity (seeTable III). Thyroglossal duct cysts were located in the midline of the oral floor, and they did not extend to the sublingual glands.

DWI findings of thyroglossal duct cysts were not similar to those of ranulas, as 2 of the 3 lesions had heterogeneous high signal intensity (see Figure 3 andTable III).

Fig. 1. Simple ranulas (A, B, C, D), plunging ranulas (E, F, G, H), and squamous cell carcinomas (I, J, K, L) on MRI.

T1WI (A), STIR (B), and DWI (C) show well-defined masses confined to the sublingual space (arrow). A homogeneous low signal is seen on T1WI, with high signals on STIR and DWI (A, B, C). The ADC map of simple ranulas (D) shows a value of 2.46£ 10¡3mm2/s. T1WI (E), STIR (F), and DWI (G) show well-defined masses in the sublingual and subman- dibular spaces (arrow). A homogeneous low signal is seen on T1WI, with high signals on STIR and DWI (E, F, G). The ADC map of a plunging ranula (H) shows an ADC value of 2.73£ 10¡3mm2/s. T1WI (I), STIR (J), and DWI (K) show unclear and irregular masses in the sublingual and subman- dibular spaces (arrow). Squamous cell carcinomas had het- erogeneous high signal intensity on STIR and DWI (I, J). The ADC map of a squamous cell carcinoma (L) shows an ADC value of 0.93£ 10¡3mm2/s. ADC, apparent diffusion coeffi- cient; DWI, diffusion-weighted imaging; MRI, magnetic reso- nance imaging; STIR, short-T1 inversion recovery; TIWI, T1 weighted imaging.

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TableIII.Statisticaldatafor68cysticmassorlesionshavingcyst-likecomponentsinandaroundthefloorofthemouth CategoryDiagnosisMean§SDvalueofADCExtentMarginIAHomT1WISTIR 10¡3mm2/s)OMSRIUMlHoHeLMHLH PseudocystsRanulas(n=35)2.59§0.313053503503503500035 Simpleranulas(n=26)2.52§0.30*2602602602602600026 Plungingranulas(n=9)2.79§0.274590909090009 CystsEpidermoidcysts(n=4)1.55§1.40y 3140404031004 Thyroglossalductcysts(n=3)2.16§0.303030121212003 BenigntumorsHemangioma(n=8)2.24§0.664462356253008 Pleomorphicadenomas(n=3)1.84§0.503021033030003 MalignanttumorsCarcinomas(n=6)1.02§0.18z1524242460006 Adenoidcysticcarcinomas(n=2)1.44§0.20z2011202020002 Inflammations(n=7)1.53§0.76z16071634007 *SignificantdifferencebetweensimpleandplungingranulasdeterminedbyusingtheStudentttest;P=.004. ySignificantdifferencesvsranuladeterminedbyusinganalysisofvariance(ANOVA);P=.010. zSignificantdifferencesvsranuladeterminedbyusing(ANOVA);P<.001. H,high;He,heterogeneous;Ho,homogeneous;Hom,homogeneity;I,irregular;IA,internalarchitecture;L,low;M,medium;MI,multilocular;MS,multiplespaces;O,onespace;R,regular;SD,standard deviation;STIR,signalintensityonSTIR;T1WI,signalintensityonT1WI;U,unilocular.

Fig. 2. Differences of simple ranulas by sex and age on DWI (A, B, C, D) and in ADC values (E, F, G, H). DWI in a 13- year-old girl with a simple ranula (A), DWI in a 14-year-old boy with a simple ranula (B), DWI in a 54-year-old woman with a simple ranula (C), and DWI in a 52-year-old man with a simple ranula (D), all show well-defined masses confined to the sublingual space (arrow). The ADC map of a 13-year-old girl with a simple ranula (E, 2.82£ 10¡3mm2/s), the ADC map of an 11-year-old boy with a simple ranula (F, 2.36£ 10¡3mm2/ s), the ADC map of a 54-year-old woman with a simple ranula (G, 2.94£ 10¡3mm2/s), and the ADC map of a 52-year-old man with a simple ranula (H, 2.45£ 10¡3mm2/s). ADC, appar- ent diffusion coefficient; DWI, diffusion-weighted imaging.

Fig. 3. The MRI findings of an epidermoid cyst (A, B, C) and a thyroglossal duct cyst (D, E, F) as examples of cystic masses or lesions that may have cyst-like components other than ranu- las on STIR (A, D), DWI (B, E), and the ADC map (C, F). The epidermoid cyst had an oval outline, unlike ranulas (A, B). The ADC map of the epidermoid cyst (C) showed a value of 1.39£ 10¡3mm2/s, lower than that of ranulas. The ADC val- ues of these cysts were significantly lower than those for ranu- las. Thyroglossal duct cysts were located in the midline of the oral floor, and they did not continue to the sublingual glands (D, E). The ADC map of the thyroglossal duct cyst (F), had an ADC value of 2.48£ 10¡3mm2/s, relatively similar to ranulas.

The ADC values of thyroglossal duct cysts were not signifi- cantly different from ranulas. ADC, apparent diffusion coeffi- cient; DWI, diffusion-weighted imaging; MRI, magnetic resonance imaging; STIR, short-T1 inversion recovery.

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On MRI, hemangiomas were multilocular and hetero- geneous and had high signal intensity on STIR, with signal voids representing phleboliths (Figure 4; see Table III). In addition, hemangiomas infiltrated into the tongue and into the muscles of the floor of the mouth, in addition to extending to the sublingual and submandibular spaces, and the margins of the masses were irregular in some cases (seeTable III). On DWI, hemangiomas showed heterogeneous high signal inten- sity (seeFigure 4 andTable III). On MRI, pleomor- phic adenomas exhibited heterogeneous high signal intensity on STIR and DWI (seeFigure 4 andTable III).

Squamous cell carcinomas and adenoid cystic carci- nomas contained solid components and had heteroge- neous high signal intensity on STIR (Figure 5; see Table III). In addition, DWI of squamous cell carcino- mas and adenoid cystic carcinomas showed heteroge- neous high signal intensity (see Figure 5 and Table III). Moreover, the margins of masses were irregular in most cases, unlike those of ranulas (seeTable III).

Inflammatory lesions (including abscesses) showed homogeneous high signal intensity on STIR and DWI, but the soft tissues around abscesses had diffuse high signal intensity on STIR and DWI (seeFigure 5 and Table III).

ADC values of DWI for ranulas and other lesions The ADC values of the ranulas and the other lesions are shown inTable III. The mean§ standard deviation (SD) ADC value of all ranulas was 2.59§ 0.31 £ 10¡3 mm2/s (seeFigures 1and2 andTable III). The ADC values of ranulas in male and female patients were 2.67 § 0.26 £ 10¡3 mm2/s and 2.50 § 0.36 £ 10¡3 mm2/s, respectively. There was no significant differ- ence between male and female patients in the ADC values of ranulas determined by using the Student t test (P = .690). In addition, no significant correlation was found between the ADC values of ranulas and age by using Pearson’s correlation coefficient (r =0.074;

P = .674). However, there was a significant difference in the ADC values between simple and plunging ranu- las, as determined by using the Student t test (P = .004) (seeFigure 1andTable III). The ADC values of sim- ple ranulas (2.52 § 0.30 £ 10¡3 mm2/s) were signifi- cantly lower than those of plunging ranulas (2.79 § 0.27£ 10¡3mm2/s).

The mean § SD ADC values of epidermoid cysts (seeFigure 3), thyroglossal duct cysts (seeFigure 3), Fig. 4. The MRI findings of a hemangioma (A, B, C) and a

pleomorphic adenoma (D, E, F) as benign tumors represent- ing lesions that may have cyst-like components other than ranulas on STIR (A, D), DWI (B, E), and the ADC map (C, F). The hemangiomas showed heterogeneous high signal intensity (A, B). The ADC map of the hemangioma (C) with an ADC value of 2.24£ 10¡3mm2/s was relatively similar to ranulas. No significant differences were found in ADC values between hemangiomas and ranulas. Pleomorphic adenomas had heterogeneous high signal intensity on STIR and DWI (D, E). The ADC map of the pleomorphic adenoma (F) had a value of 1.35£ 10¡3 mm2/s, lower than that of ranulas, although no significant differences were found in ADC val- ues between these lesions. ADC, apparent diffusion coeffi- cient; DWI, diffusion-weighted imaging; MRI, magnetic resonance imaging; STIR, short-T1 inversion recovery.

Fig. 5. The MRI findings of a squamous cell carcinoma (A, B, C), an adenoid cystic carcinoma (D, E, F), and inflam- mation (G, H, I) as examples of lesions with cystic compo- nents on STIR (A, D, G), DWI (B, E, H), and the ADC map (C, F, I). The ADC map for the squamous cell carcinoma (C) showed an ADC value of 0.92£ 10¡3mm2/s. The ADC map of the adenoid cystic carcinoma (F) had an ADC value of 1.10£ 10¡3 mm2/s. The ADC map of the inflammatory lesion (I) had an ADC value of 1.52£ 10¡3 mm2/s. The ADC values for these lesions were significantly lower than those of ranulas in this study. ADC, apparent diffusion coeffi- cient; DWI, diffusion-weighted imaging; MRI, magnetic reso- nance imaging; STIR, short-T1 inversion recovery.

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hemangiomas (seeFigure 4), pleomorphic adenomas (seeFigure 4), squamous cell carcinomas (seeFigure 5), adenoid cystic carcinomas (see Figure 5), and inflammation (seeFigure 5) were 1.55§ 1.40 £ 10¡3 mm2/s, 2.16 § 0.30 £ 10¡3 mm2/s, 2.24 § 0.66£ 10¡3 mm2/s, 1.84 § 0.50 £ 10¡3 mm2/s, 1.02

§ 0.18 £ 10¡3mm2/s, 1.44§ 0.20 £ 10¡3mm2/s, and 1.53 § 0.76 £ 10¡3 mm2/s, respectively. There were significant differences in the ADC values between ranulas and epidermoid cysts (P = .010); malignant tumors, including squamous cell carcinomas and ade- noid cystic carcinomas (P< .001); and inflammation (P< .001), as determined by using the Student t test.

However, no significant differences in ADC values were found between ranulas and thyroglossal duct cysts, hemangiomas, or pleomorphic adenomas.

DISCUSSION

The most interesting result of the present study are the characteristic findings of DWI and ADC values in ranulas elucidated for the first time. There was a signif- icant difference in the ADC values between simple and plunging ranulas (P = .004). However, there was no significant difference between males and females in the ADC values of ranulas and no significant correla- tion between the ADC values of ranulas and age. Gen- der and age would, therefore, not alter the validity of using MRI findings, including DWI and ADC values, clinically for the differential diagnosis of cystic masses or lesions that may have cyst-like components in and around the floor of the mouth.

As expected, there was a significant difference in ADC values between simple and plunging ranulas.

The ADC values of simple ranulas were significantly lower than those of plunging ranulas. A possible expla- nation for this difference is that the fluid in simple ranulas has a higher viscosity because of higher con- centrations of protein and other substances. Ranulas with higher ADC values tend to expand into the sub- mandibular spaces if the ranulas are limited to the sublingual spaces.

On DWI, ranulas could be consistently identified as homogeneous, high-signal areas, with relatively less distortion and blurring. The reason for the conspicuity of the lesions on DWI was probably the unrestricted water proton mobility that was of a greater degree than the “T2 shine-through effect.” The ADC values in ranulas were relatively high, at 2.59 § 0.31 £ 10¡3 mm2/s. The ADC values of ranulas may reflect the rela- tive free water volume and low protein concentration.

Lower protein concentration in ranulas decreases fluid viscosity and increases water proton mobility. The ADC values of ranulas in the present study were higher than those of normal tissues on ADC maps because the

mobility of water protons was relatively freer than in other circumstances.

In comparison, all other masses had one or more MRI, DWI, and ADC values different from those of ranulas in the present study. It was possible to objec- tively differentiate between ranulas and other cystic masses or lesions that may have cyst-like components because there was a significant difference between the ADC values of ranulas and those of the other lesions.

As a concrete example, because the ADC values of ranulas were significantly higher, by using ADC values we could differentiate ranulas from epidermoid cysts;

malignant tumors, including squamous cell carcinomas and adenoid cystic carcinomas; and inflammation. The MRI findings of ranulas, as reported by Kurabayashi et al.,6helped us differentiate ranulas from cystic masses or lesions that may have cyst-like components in the present study. In our investigation, the characteristics of the extent, margin, internal architecture, homogene- ity of masses on DWI, and signal intensity on T1WI and STIR in the respective masses were relatively simi- lar to those reported by Kurabayashi et al.6However, we considered that it would be very valuable to objec- tively differentiate ranulas from other cystic masses or lesions that may have cyst-like components by using ADC values.

When only the ADC values were used, we found ranulas to be similar to thyroglossal duct cysts, heman- giomas, and pleomorphic adenomas. Therefore, other MRI findings should also be considered when formu- lating a differential diagnosis. The characteristic MRI findings of thyroglossal duct cysts included location in the midline of the oral floor and lack of extension into the sublingual glands, unlike the findings in ranulas. In addition, unexpectedly, the internal architecture of some thyroglossal duct cysts was heterogeneous.

Hemangiomas were multilocular in many cases and occasionally heterogeneous on STIR and/or DWI, unlike in ranulas. The ADC values of squamous cell carcinomas and adenoid cystic carcinomas in this investigation were similar to those of a previous report.10The possible explanation is that the ADC val- ues in cases of massive lesions with solid components are lower than they are in the completely cystic masses.

However, to the best of our knowledge, there have been no reports of ADC values of other cystic masses or lesions that may have cyst-like components in and around the oral floor. Data from previous reports of ADC values of nonoral lesions were compared with the present study’s data on masses in and around the oral floor. Our data on epidermoid cysts were similar to those from a previous study of the brain, and the ADC values of hemangiomas in the liver were also similar to those of the present study’s data.11-14In addi- tion, the present study’s data on pleomorphic adenomas

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were similar to those from a previous report on parotid glands.15Therefore, the DWI and ADC values in the present study were acquired appropriately, and this is the first report of ADC values and DWI findings of cys- tic masses or lesions that may have cyst-like compo- nents in and around the floor of the mouth, in addition to those of ranulas, which makes the findings of this study very valuable.

The acquisition of DWI and ADC values is noninva- sive, requiring no additional treatment, and the time needed for the acquisition of DWI and the production of ADC maps is about 5 minutes. Therefore, DWI and ADC values should always be acquired for the diagno- sis of cystic masses or lesions with cyst-like compo- nents of the floor of the mouth.

It has been previously shown that DWI could be of value in the diagnosis of similar conditions in the oral and maxillofacial regions.9,16 Commonly used in examinations of the brain, the discovery and clinical use of parallel-imaging in DWI can be also effective in examining the oral and maxillofacial regions with their complex anatomy because of the improved spatial res- olution and the decrease in susceptibility artifacts.17,18 The use of DWI and ADC values in the diagnosis of abnormalities in the oral and maxillofacial regions will likely increase.

This study has several possible limitations. First, because the sample size was relatively small, the ADC values might not be standard and accurate. In particu- lar, the various cystic masses or lesions that may have cyst-like components in and around the oral floor other than ranulas were few in number. Therefore, more patients must be evaluated to produce standardized ADC values for certain lesions, such as lymphangioma, lipomas that may be similar to cystic masses on palpa- tion, dermoid cysts, and so on, in the oral and maxillo- facial regions. At the same time, surgical procedures were not performed for some benign cysts and benign tumors, such as epidermoid cysts, thyroglossal duct cysts, hemangiomas, and ranulas. However, such lesions can be diagnosed appropriately on the basis of findings from clinical and imaging examinations. In addition, data by gender and age might not necessarily be accurate in the present study sample. In our next study, we plan to evaluate larger sample sizes to con- firm the present results and establish the diagnostic cri- teria for cysts and cyst-like lesions in and around the floor of the oral cavity.

CONCLUSIONS

The characteristic DWI findings and ADC values of ranulas were evaluated, and the data on ranulas and other masses in and around the floor of the mouth, including epidermoid cysts, thyroglossal duct cysts, hemangiomas, pleomorphic adenomas, squamous cell

carcinomas, adenoid cystic carcinomas, and inflamma- tory lesions, were compared. Ranulas were all well- defined, homogeneous masses with high signal inten- sity on DWI. The mean§ SD ADC of 35 ranulas was 2.59§ 0.31 £ 10¡3mm2/s. There was a significant dif- ference in ADC values between simple and plunging ranulas. On DWI, most cystic masses or lesions that may have cyst-like components were heterogeneous, and most ADC values were significantly lower than those of ranulas. However, the ADC values of thyro- glossal duct cysts and hemangiomas were similar to those of ranulas, although the MRI findings were quite different. The characteristic DWI findings and ADC values of ranulas should be determined accurately, and the data, as those from the present study, will be very useful in the differential diagnosis of many kinds of diseases in and around the floor of the mouth.

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Reprint requests:

Yasuhiro Morimoto, Division of Oral and Maxillofacial Radiology, Kyushu Dental University, 2-6-1 Manazuru, Kokurakita-ku, Kita- kyushu 803-8580, Japan.

rad-mori@kyu-dent.ac.jp

參考文獻

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